A. Field of the Invention
The present invention relates to a CRT (Cathode Ray Tube), and more particularly to a structure of a magnetic shield provided in a CRT to shield against the earth magnetic field and the external magnetic field.
B. Discussion of the Related Art
Referring to FIGS. 1 and 2, a flat CRT comprises a panel 2 provided on the front surface of the CRT. A fluorescent screen 9, which is coated with a fluorescent substance, is provided on the inner surface of the panel 2 to emit visible light when struck by electron beams. A funnel 3 is fixedly attached to the rear of the panel 2 by fusion with a frit glass. An external shield 6 is coupled to the CRT by a reinforcement band 10. The band 10 is provided on a connection member of the panel 2 and the funnel 3.
An electron gun 5 is provided in a neck 3a of the funnel 3 to emit three electron beams of three primary colors, red, green and blue. A shadow mask 7 is mounted on the inner surface of the panel 2 and perforated with numerous tiny holes of slit shapes to ensure that each of the three electron beams strikes only its intended color fluorescent dot. A frame 8 is provided to separate the shadow mask 7 from the panel 2 at a distance.
The electron beams 4, which are emanated from the electron gun 5, pass through respective holes of the shadow mask 7 and strike the fluorescent screen 9, which is coated with a fluorescent substance on the inner side of panel 2, to emit visible light. The external shield 6 reduces the extent of the deflection defocusing of the electron beams which may be caused by the earth magnetic field and/or the external magnetic field.
Referring to FIG. 3, the external shield 6 comprises a funnel flange 6-1 and a panel flange 6-2. The funnel flange 6-1 extends along the circumference of the funnel 3 towards the neck 3a of the funnel 3. The panel flange 6-2 provides contact with the panel 2. Since the external shield 6 is made of magnets, the external magnetic field introduced into the CRT tends to easily flow through the external shield 6, reducing the above-described deflection defocusing effect caused by the external magnetic field.
Referring to FIG. 2, a portion A refers to a situation in which the panel flange 6-2 of the external shield 6 mounted on the CRT tends to cover a portion of the viewing screen of the panel 2. In this situation, the panel flange 6-2 must be cut short, as shown in FIG. 1, after the external shield 6 is mounted on the CRT. This cutting process is cumbersome and inconvenient. Further, the external shield 6 is typically thin, and therefore when it is mounted on the outside of the CRT, it tends to be deformed.
Yet further, the external shield 6 is a major cost factor in making CRTs because it has to be drawn along the shape of the funnel during manufacturing.